Conventionally, electrolytic capacitors and multilayer ceramic capacitors have been well known. Electrolytic capacitors comprise valve metals such as Al and Ta, while multilayer ceramic capacitors comprise electrodes of, for example, Pd or Ni and dielectrics of BaTiO.sub.3 or the like. Most of these capacitors, which are used for most electric circuits such as supplies, have two electrode terminals. According to the recent trend for requirement for smaller electric circuits that can be operated at high frequencies operating circuits, capacitors are also required to have high capacitance and low impedance. Regarding power supply circuits for driving CPU of computers or switching power supply circuits, particularly, higher noise rejecting property and ripple current absorbing property are required to design circuits that will be operated at high frequencies. As a result, low impedance capacitors with low equivalent series resistance (ESR), low equivalent series inductance (ESL), against high ripple current absorbing property and high capacitance are demanded highly. To meet the requirements, especially for obtaining low ESR, electroconductive polymers with high electric conductivity have been studied and developed to be used for solid electrolytes for cathode (hereinafter, cathodic solid electrolytes) of electrolytic capacitors.
The structure of a conventional aluminum electrolytic capacitor is explained below with a reference to FIG. 10. A capacitor element is manufactured by the steps of:
preparing an anodic foil 81 by roughening and forming a dielectric layer on the surface and also preparing a surface-roughened current-collecting cathodic foil 82; PA1 arranging separators 83 between the anodic foil 81 and the current-collecting cathodic foil 82; and PA1 winding the anodic foil 81, the current-collecting cathodic foil 82 and the separators 83. This element is sealed in a case with an electrolytic solution. Leads 84 as electrode terminals are connected respectively with the anodic foil 81 and the current-collecting cathodic foil 82.
The structure of a conventional multilayer ceramic chip capacitor is explained below with a reference to FIG. 11. Electrode layers 91 comprising sintered bodies of Pd, Ni or the like and dielectric layers 92 are laminated alternately. The electrode layers 91 are connected alternately with electrode terminals 93.
The structure of a conventional tantalum electrolytic capacitor with electroconductive polymer is explained below with a reference to FIGS. 12(a) and 12(b). FIG. 12(a) is a cross-sectional view showing the structure of a conventional tantalum electrolytic capacitor with electroconductive polymer, and FIG. 12(b) is an expanded cross-sectional view partially showing the configuration of a capacitor element. A capacitor element 101 is prepared by forming a dielectric layer 101b on the surface of a tantalum powder sintered body 101c, and subsequently forming on the dielectric layer 101b an electroconductive polymer layer 101a. The electroconductive polymer layer 101a acts as the true cathode, and it is connected to a cathodic terminal 102 via an electroconductive adhesive layer 103. The anodic terminal 104 is connected to a lead 105 from the sintered body 101c. The element including these members is encased with a mold resin layer 106.
In addition to that, decreasing inductance value is further required to lower the impedance at high frequencies of about 100 kHz or more. Regarding this requirement, four-terminal capacitors (capacitors with four electrode terminals) are disclosed, for example, in Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 6-267802, Tokkai-Hei No. 6-267801, and "SP-cap" (a trademark of Matsushita Electric Industrial Co., Ltd. see Proceeding of '92 Symposium on Switching Power Supply System (S6(1994)-1-1)). On the other hand, there is a need for development of capacitors to allow relatively high current to the primary or secondary side of a power supply, while meeting the requirement for operation at high frequencies. An invention of a capacitor to decrease the entire impedance and raise the current-carrying capacity is also disclosed by Tokkai-Hei No. 4-32214.
The above-mentioned aluminum electrolytic capacitor, however, has some disadvantages, including its high impedance due to the use of an electrolytic solution comprising ethylene glycol or the like as the main solvent, and its high inductance components due to the wound electrode foil. Although a conventional tantalum electrolytic capacitor lowers ESR by using electroconductive polymers for the electrolyte, high capacitance cannot be obtained sufficiently. A conventional multilayer ceramic chip capacitor cannot obtain high capacitance in comparison with a conventional aluminum electrolytic capacitor. In conventional techniques where a four-terminal structure is adapted to lower ESL flowering the inductance value), sufficient capacitance has not been obtained. In addition, the capacitor itself will generate heat and fail at the primary or secondary side of a power supply where relatively high current from about several A to several dozens of a flows. Considering these disadvantages, such a conventional capacitor cannot be used to pass a high current while meeting the requirement for circuits to be operated at high frequencies.
The reasons are as follows. In a conventional aluminum electrolytic capacitor prepared by winding a slender electrode foil, the resistance of the foil becomes high and the capacitor element will generate heat easily even if a four-terminal structure is used. A conventional tantalum electrolytic capacitor also can lower the ESR to some degree by using functional polymers. It is not easy, however, to raise capacitance per volume to provide high capacitance because a sintered body is used, and a four-terminal structure is difficult to obtain. A multilayer ceramic capacitor disclosed in Tokkai-Hei No. 4-32214 adapts a four-terminal structure to lower ESL, and increases current-carrying capacity by providing double electrode layers. For the manufacturing process, the material for the electrode layers should be a sintered metal with a thickness of several .mu.m. Therefore, the available current value is limited to several amperes, so the lamination number should be increased to be used for the primary or secondary side of the power supply or the like where relatively high current will flow. Increasing the lamination is not easy in the manufacturing steps, or the volume per capacitance will be increased if many electrode layers are laminated. The electrode layer cannot be made thicker than 3 .mu.m substantially, since delamination will occur (the dielectric layer peels off from the electrode layer) in manufacturing.
These problems are explained below with a reference to FIGS. 13 and 14. FIG. 13 shows an equivalent circuit of a conventional two-terminal capacitor (inside the dotted box). FIG. 14 is an equivalent circuit diagram to show problems for a conventional four-terminal capacitor (inside the dotted box). To provide a capacitor that can be operated at high frequencies, ESR (equivalent series resistance) 111 and ESL (equivalent series inductance) 112 should be lowered. ESR can be lowered by using electroconductive polymers for the electrolyte or by improving the collector. ESL can be lowered by providing a four-terminal structure as shown in FIG. 14. In the conventional four-terminal capacitor in FIG. 14, however, the impedance as a capacitor element is high, and the resistance R+ (the resistance of anode) 121 and R- (the resistance of cathodic collector) 122 acting as circuit wires greatly contribute to heat generation when current flows. Thus such a structure cannot be used for a circuit of a primary or secondary side of a power supply where a relatively high current will flow. Tb meet the requirement, any means to decrease the R+ 121 and R- should be taken.
As mentioned above, conventional capacitors cannot meet a requirement for high capacitance and a low impedance. When such a capacitor is used for a circuit operated at high frequencies at a primary or secondary side of a power supply where a relatively high current is flowing, the element generates much heat, and the applicable current will be restricted.